The linear variable differential transformer (LVDT) is a commonly-used linear position transducer that includes a movable magnetic core, a primary winding and two secondary windings. Since there is no contact between the core and the windings, there is no friction and no mechanical wear to limit the life of the transducer. This is especially important in high reliability applications and in hostile environments. As an example, the control surfaces of aircraft exhibit vibration which would quickly destroy a mechanical contact-type transducer. The position of the magnetic core determines the voltage induced on each of the two secondary windings. When the core is approximately centered in the secondary windings, an equal voltage is induced on each secondary winding. As the core is displaced from the center, or null point, the voltage induced on one secondary winding increases while the voltage on the other secondary winding decreases. The two secondary windings are usually connected in series opposing, and the resulting difference voltage is measured. The phase relative to the primary voltage indicates the direction relative to the null point. In this scheme, the primary drive voltage is a scale factor that directly affects the output voltage, and must be stabilized.
Another prior art detection scheme for LVDT's employs synchronous full wave detection. Since the output voltage goes to zero at the null point, the reference signal for synchronous detection must be derived from the primary drive signal. Since there is usually a phase shift between the primary and secondary signals, a compensating phase shift must be added to the reference signal. The required phase shift complicates the detection technique, and errors are introduced if the phase shift is incorrectly compensated. In this scheme, the scale factor is also sensitive to amplitude variations of the primary drive signal.
In the past, the interface circuitry for LVDT's was mounted on printed circuit boards and required various adjustments, thereby making LVDT's relatively inconvenient to use as position transducers. It is desirable to incorporate the drive and detection circuitry for LVDT's into a module or integrated circuit that provides a voltage representative of core position. Such an integrated circuit should be highly accurate and should be adaptable for use with a variety of different LVDT types. In addition, it should have a minimum of required external components, particularly variable components and active components, and should be easy to use.
An integrated LVDT interface circuit is described by Nicholas C. Gray in "Simplifying LVDT Signal Conditioning," Machine Design, May 7, 1987, pp. 103-106 and by Zahid Rahim, "LVDT Interface Chip's Functional Blocks Offer Versatility," EDN, May 29, 1986, pp. 159-168. The interface circuit described in those references utilizes the traditional technique of synchronous detection. An external voltage reference and an external adjustment of the reference signal phase are required. Since synchronous detection is utilized, the above-described sensitivity to variations in primary drive voltage is present. In addition, a change in phase shift from the primary to the secondary or an error in the phase shift network represents a scale factor error.
An LVDT interface circuit having a binary encoded output is described by Daniel Denarc in "Transducer Converters Ease Industrial Measurements," Electronic Design, Sep. 4, 1986, pp. 118-124. The disclosed interface circuit utilizes a ratiometric closed loop conversion technique. Another digital LVDT interface circuit is described in DDC News, October 1987.
It is desirable to provide an LVDT interface circuit that is insensitive to primary voltage variations, that has a scale factor and offset which are relatively insensitive to temperature variations and that has substantially better linearity than the LVDT transducer. Furthermore, the circuit must be small in size, have a minimum of external components and be convenient to use.
It is a general object of the present invention to provide a novel monolithic interface circuit for linear variable differential transformers.
It is another object of the present invention to provide an LVDT interface circuit which generates an analog output voltage that is a highly accurate representation of core position.
It is yet another object of the present invention to provide an LVDT interface circuit having a scale factor, which relates output voltage to LVDT core position, that is highly stable as a function of ambient temperature.
It is still another object of the present invention to provide an LVDT interface circuit that has an output voltage which is substantially insensitive to variations in the primary drive voltage.
It is a further object of the present invention to provide an LVDT interface circuit wherein a single passive component determines the scale factor which relates output voltage to LVDT core position.
It is a further object of the present invention to provide an LVDT interface circuit that has substantially better transfer function linearity than conventional LVDT interface circuits.
It is a further object of the present invention to provide an LVDT interface circuit that can selectably generate an output which is the integral of core position for closed loop applications.
It is a further object of the present invention to provide an LVDT interface circuit in which the primary drive signal is easily programmable in amplitude and frequency.
It is another object of the present invention to provide a monolithic integrated LVDT interface circuit that is compact and requires a minimum of external components.
It is another object of the present invention to provide a decoder for processing a pair of signals of equal frequency to determine the ratio of amplitudes.